The present invention relates generally to refrigeration systems, and, more specifically, to refrigerant level control in a phase separator contained therein.
A typical refrigeration system includes a compressor for compressing a normally gaseous refrigerant into high pressure gas disposed in serial flow communication with a condenser for removing heat therefrom and condensing the refrigerant to its liquid phase; an expansion device; and an evaporator located in a refrigerator housing for providing cooling therein in a conventional manner. The refrigeration system may operate on a simple one stage vapor compression cycle or may operate in a more complex cycle such as a dual evaporator two stage cycle.
In the two stage cycle, a pair of compressor stages are disposed in turn upstream of the condenser, and a pair of evaporators are disposed in turn downstream of the condenser. A separate expansion device is disposed upstream of each of the evaporators, and a phase separator is disposed between the first evaporator and the second expansion device. The phase separator is a relatively simple vessel which effects phase separation by gravity, with the refrigerant liquid filling the bottom portion of the vessel, and refrigerant vapor filling the upper portion of the vessel. And, the level of the liquid varies during operation and is acceptable over a wide range of levels as long as two phase refrigerant is maintained in the separator vessel.
The vessel has an inlet at its top for receiving the refrigerant from the first evaporator and an outlet at its bottom for channeling the liquid to the second expansion device. The separator also has a second outlet positioned at a suitable vertical elevation above the normal range of liquid level in the vessel which is disposed in flow communication between the two compressor stages for returning a portion of the vapor for undergoing compression in the second stage.
Typical expansion devices are relatively simple capillary tubes which have known limitations. An improved expansion device is a conventional electrical solenoid valve which is suitably controlled by a pulse width modulated signal between a fully open position and a fully closed position, with the valve typically being a normally closed valve when unenergized. Pulse width modulation is conventionally accomplished using a specific frequency in the exemplary range of about 0.1-2 Hz, with the average flowrate through the valve being controlled by the duty cycle of the pulse width modulation which is simply the duration or length of each pulse per cycle for which the valve is open.
Various pulse width modulation control systems are known for controlling operation of the refrigeration system based on various parameters such as superheat, dryness, and subcooling in various components of the system which in turn may be determined by measuring various temperatures in the system or the liquid level in the phase separator. It is known to control pulse width modulation by measuring liquid level in the phase separator using one or two level sensor switches therein. However, it is desirable to sense the phase separator liquid level in a simple and inexpensive manner for keeping the cost of the refrigeration system correspondingly low while obtaining a suitable useful life and effective pulse width modulation.